For a relatively long time, use has been made of ion exchangers for removing valuable metals and heavy metals such as tin, cobalt, nickel, copper, zinc, lead, uranium, bismuth, vanadium, elements of the platinum group such as ruthenium, osmium, iridium, rhodium, palladium, platinum, and also the noble metals gold and silver, in particular from aqueous solutions. For this purpose, in addition to cation exchangers or anion exchangers, use is also preferably made of chelate resins.
The use of chelate resins for removing heavy metals or valuable metals is described, for example, in R. Hering, Chelatbildende Ionentauscher [Chelate-forming Ion Exchangers], Akademie Verlag, Berlin, 1967, pages 150 to 157. Mention is made, inter alia, of chelate resins containing iminoacetic acid groups. Chelate resins in many cases exhibit a significantly higher selectivity for heavy metals than, for example, cation exchangers containing strongly acidic sulphonic acid groups.
The customary ion exchangers take up heavy metals from aqueous solutions at pH>4. At pHs less than approximately 4, the selectivity of chelate resins for heavy metals decreases, since their functional groups are protonated. However, the removal of heavy metals from solutions or suspensions having strongly acid pHs in the range from about 4 to about 1 is of considerable technical interest.
For the recovery of heavy metals, rocks are treated with sulphuric acid. The valuable metals are dissolved out of the rock and are present in the strongly acidic rock-sulphuric acid suspension. In addition to the valuable metals, the rocks frequently also contain iron which is frequently present in dissolved form as iron 3+ ion. Ion exchangers take up iron 3+ ions readily without, however, subsequently releasing them to the same extent on regeneration of the ion exchanger. Iron ions therefore block the exchange capacity of the ion exchanger.
Ion exchangers are sought which can take up valuable metals from acidic solutions or suspensions in the pH range from 4 to about 1 considerably more selectively than iron ions.
U.S. Pat. No. 4,098,867 and U.S. Pat. No. 4,031,038 describe chelate resins which bear methylaminopyridine groups.
They are produced by halomethylating polymer beads based on styrene and divinylbenzene, wherein, on average, 0.1 to 1.0 halomethyl groups are introduced per aromatic ring as a reactive group for adding the aminomethylpyridine chelate functionality.
Restricting the degree of halomethylation of the polymer beads also restricts the amount of aminomethylpyridine groups in the chelate resin and therefore the exchange capacity of the chelate resin.
The halomethylation method described in U.S. Pat. No. 4,098,867 for introducing the functional group has disadvantages which lead to a restriction of the degree of functionalization. The disadvantages are described in EP-A 0 481 603. For instance, on halomethylation, post-crosslinking occurs which leads to a loss of halomethyl groups. Owing to the resultant loss of halomethyl groups which could be reacted with aminomethylpyridines, the resultant chelate resins have fewer functional groups available for recovering valuable metals, which considerably limits their use in metallurgy. It was an object of the present invention to provide a highly functionalized high-capacity chelate resin which is stable even at low pHs which effectively adsorbs valuable metals from acidic aqueous solutions even in the presence of iron ions.